This invention generally relates to centrifugal compressors, and more particularly to an apparatus for achieving passive damping of flow disturbances in a centrifugal compressor to control compressor surge.
The operating range of turbomachinery compression systems, such as centrifugal compressors, is very often limited by the onset of fluid dynamic instabilities such as choke and surge. Choke is determined by sonic velocity (Mach Number) limits. Surge is a self-excited instability, evidenced by large amplitude oscillations of annulus-averaged mass flow and plenum pressure rise. Surge can cause reduced performance and efficiency of the turbomachine, and, in some cases, failure due to the large unsteady aerodynamic force on the various turbomachinery components.
To avoid surge, the compression system is generally operated away from the "surge line" which is the boundary between stable and unstable compression system operation, and which is graphically portrayed in FIG. 1. It is known that operating the compressor at some distance from this surge line, on the negatively sloped part of the compressor speed line of FIG. 1, can ensure stable compressor operation. Doing this, however, may result in a performance penalty since peak performance and efficiency often occur near the surge line.
If the surge line can be adjusted to include lesser flow rates, a number of operational advantages are possible. These operational advantages include, but are not limited to, providing added reliability since the likelihood of surge induced damage will be decreased, operating the compressor with lower power consumption by operating the compressor at or closer to its peak efficiency point, and providing compressor operation over a wider range of operating capacities and pressures.
Because of its importance, the control of compressor surge has been investigated in the past. For example, active suppression of centrifugal compressor surge has been demonstrated on a centrifugal compressor equipped with a servo-actuated plenum exit throttle controller. This technique teaches using closed-loop feedback control of the dynamic behavior of the compression system.
Additionally, U.S. Pat. No. 5,199,856 teaches a surge control system comprising coupling a centrifugal compressor system to a flexible plenum wall which is modeled as a mass-spring-damper system to respond to pressure perturbations in the plenum. The flexible plenum wall is described as a rigid piston which is sealed with a convoluted diaphragm.
The surge control systems described hereinabove generally require components and assemblies in addition to the standard components of turbomachinery compression systems. The present invention provides a passive surge control system which is made integral with standard centrifugal compressor components thereby eliminating the need for additional compressor components and assemblies.